Method of driving light emitting element array

Abstract

An exposure system is provided with a light emitting element array formed by a plurality of light emitting elements formed at the intersections of anodes and cathodes arranged in matrix so that a photosensitive material is exposed to an image formed on the light emitting element array. A method of driving the light emitting element array includes the steps of driving the light emitting elements in constant-current drive before and / or during an exposure period, measuring and storing in a memory means the anode voltage of each light emitting element at that time, and driving the light emitting elements in constant-voltage drive at a voltage equal to the measured anode voltage at least during beginning of the subsequent exposure periods.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]This invention relates to a method of driving a light emitting element array such as an organic EL (electroluminescent) element array.[0003]2. Description of the Related Art[0004]Conventionally, there has been known an exposure system which comprises a light emitting element array comprising a plurality of two-dimensionally arranged light emitting elements, such as organic EL elements, and a drive circuit which controls the light emitting time (light emitting pulse width) of each of the light emitting elements on the basis of an image data carrying thereon a gradation image and exposes a photosensitive material to an image formed on the light emitting element array on the basis of the image data. An example of such an exposure system is disclosed in U.S. patent Laid-Open No. 20010052926.[0005]As a representative of the light emitting element array, there have been known those of a system so-called a simple matrix system...

AI Technical Summary

Benefits of technology

The present invention provides a method for driving a light emitting element array for an exposure system that can achieve excellent rise-up characteristics and suppress fluctuation in response and / or light emission of the light emitting elements. The method involves driving the light emitting elements in constant-current drive before and / or during an exposure period, measuring the anode voltage of each light emitting element, and driving the light emitting elements in constant-voltage drive at a voltage equal to the measured anode voltage at least during the beginning of the subsequent exposure periods. By doing so, the method can provide stable and high-quality exposure images.

Problems solved by technology

The former is excellent in response but is poor in stability due to drop and fluctuation of the forward voltage by change with time of the environment of use or each light emitting element.

However, when the constant-current drive system is employed for the light emitting element array of the simple matrix system, there has been known a problem that the rise-up characteristics are bad.

However due to existence of the parasitic capacity described above, all capacities of the light emitting elements formed on the anode are charged with the constant current when the scanning electrodes are switched and accordingly, it requires a long time for the light emitting element to emit light after its capacity is charged, which deteriorates the rise-up characteristics.

However the anode voltage of each light emitting element upon the constant-current drive is not always constant and the anode voltages of the light emitting elements in one array fluctuate according to the initial difference and / or the change with time of the light emitting elements.

Such fluctuation and / or difference of the anode voltage lead to fluctuation of the rise-up time.

Though the fluctuation in the light emission does not give rise to a significant problem when the light emitting element array is used as a display means, the fluctuation in the light emission causes deterioration of the quality the exposed image when the light emitting element array is used as an exposure head.

Especially, the difference in the light emission between the elements causes a scoring unevenness extending in a sub-scanning direction to greatly deteriorate the quality of the exposed image when the exposure head and the photosensitive material are moved in the sub-scanning direction (sub-scanning).

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